Origin of Resting Membrane Potential in a Nerve

The resting membrane potential is established by the diffusion potentials, membrane permeability and electrogenic nature el the Na+ – K+ pump. It is mostly determined by the concentrations of the ions in the fluids on both sides of the cell membrane and the ion transport proteins that are in the cell membrane.

• Potassium diffusion potential- A high ratio of conc. K+ from inside to outside of the cell 35 to 1, produces a Nernst potential of -94 millivolts according to Nernst equation.
• Sodium diffusion potential- The ratio of conc. at Na+ from inside to outside the membrane is 0.1 and this gives a calculated Nernst potential of + 61 millivolts.

Membrane permeability

The permeability of the nerve fiber membrane to potassium is about 100 times as great as that of sodium, so that diffusion of potassium contributes to more to the membrane potential. The use of this high value of permeability in the Goldmann equation gives an internal membrane potential of -86 millivolts.

Electrogenic Nature of Na+ – K+ pump

The Na+ – K+ pump is electrogenic because it produces a net deficit of positive ions inside the cell. This causes a negative charge of about -4 millivolts in the cell membrane.

Therefore the net membrane potential with all these factors operative at the same time is about -90 millivolts.

Nernst Potential

The potential level across the membrane that prevents net diffusion of an ion in either direction through the membrane is called the “Nernst Potential” for that ion.

To calculate it following equation is used-

EMF (mV) = ± 61 log = [(conc. inside) / (conc. outside)]

It is the ‘Nernst Equation’.

In summary, the diffusion potentials alone caused by K+ and Na+ diffusion would give a membrane potential of about -86 mV, almost all of this being determined by K+ diffusion. Then, an additional -4 mV is contributed to the membrane potential by the electrogenic Na+ – K+ pump, giving a resting membrane potential of – 90 mV.

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